Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Thu, 11 Jul 2002 21:07:13 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Thu, 11 Jul 2002 21:07:12 -0400 Received: from gateway-1237.mvista.com ([12.44.186.158]:60924 "EHLO av.mvista.com") by vger.kernel.org with ESMTP id ; Thu, 11 Jul 2002 21:07:11 -0400 Message-ID: <3D2E2C48.DCB509D7@mvista.com> Date: Thu, 11 Jul 2002 18:09:28 -0700 From: george anzinger Organization: Monta Vista Software X-Mailer: Mozilla 4.77 [en] (X11; U; Linux 2.2.12-20b i686) X-Accept-Language: en MIME-Version: 1.0 To: Stevie O CC: lkml Subject: Re: HZ, preferably as small as possible References: <5.1.0.14.2.20020711201602.022387b0@whisper.qrpff.net> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2142 Lines: 57 Stevie O wrote: > > At , [] wrote: > > > > A lot of people are talking about how HZ needs to be a constant, etc. > > I don't do much kernel hacking, so allow me to post a query that would (probably) better belong on #kernelnewbies if I wasn't so damn lazy ;) -- > > Why must HZ be the same as 'interrupts per second'? Well, in truth it has nothing to do with interrupts. It is just that that is the way most systems keep time. The REAL definition of HZ is in its relationship to jiffies and seconds. I.e. jiffies * HZ = seconds, by definition. Then we define interfaces that promise to return so many jiffies from now and we keep execution time and time slice times in jiffies. In order to keep these things true, it is usual to set up some sort of timer to interrupt once each jiffie. Now we can actually do this two ways. We can say that the interrupt is a reminder to look at a "reliable clock" and update the system time with what we find OR we can use the interrupt to actually drive the system time. The former is the more accurate way of doing things as it eliminates interrupt latency. It also allows us to use a more sloppy source of interrupts since they are just reminders to check a clock and not actually driving the clock. This, by the way, is what the high-res-timers patch does. Doing things this way also allows one to reprogram the timer interrupt hardware with out worrying too much about loosing track of time. The HRT patch does this to generate interrupts at sub jiffie intervals, but only when required. -g > > -- > Stevie-O > > Real programmers link their executables by hand. > -- George Anzinger george@mvista.com High-res-timers: http://sourceforge.net/projects/high-res-timers/ Real time sched: http://sourceforge.net/projects/rtsched/ Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/